RESISTANCE SPOT WELDING METHOD AND WELD MEMBER PRODUCTION METHOD

Abstract

Provided is a resistance spot welding method wherein main current passage includes two or more electrode force application steps including a first electrode force application step and a second electrode force application step following the first electrode force application step, an electrode force F.sub.1 in the first electrode force application step and an electrode force F.sub.2 in the second electrode force application step in the main current passage satisfy a relationship F.sub.1<F.sub.2, and an electrode force switching point T.sub.f from the first electrode force application step to the second electrode force application step in the main current passage is set to satisfy predetermined relational formulas.

Claims

1. A resistance spot welding method of squeezing, by a pair of electrodes, parts to be welded which are a plurality of overlapping metal sheets and passing a current while applying an electrode force to join the parts to be welded, wherein main current passage includes two or more electrode force application steps including a first electrode force application step and a second electrode force application step following the first electrode force application step, and an electrode force F.sub.1 in the first electrode force application step and an electrode force F.sub.2 in the second electrode force application step in the main current passage satisfy a relationship F.sub.1<F.sub.2, and an electrode force switching point T.sub.f from the first electrode force application step to the second electrode force application step in the main current passage is set to satisfy the following Formulas (1) to (3): in the case where T.sub.A≤0.8×T.sub.0,
T.sub.A≤T.sub.f<T.sub.0   (1) in the case where 0.8×T.sub.0<T.sub.A≤T.sub.0 or 0.9×R.sub.0≤R.sub.A≤R.sub.0,
9×T.sub.0<T.sub.f<1.1×T.sub.0   (2) in the case where R.sub.A<0.9×R.sub.0,
T.sub.0<T.sub.f≤T.sub.0+2×(R.sub.0−R.sub.A)/R.sub.0×T.sub.m   (.sup.3) where T.sub.0 is a reference electrode force switching point from the first electrode force application step to the second electrode force application step, T.sub.m is a total welding time in the main current passage, R.sub.A is a time integration value of a resistance between the electrodes from current passage start of the main current passage to the reference electrode force switching point T.sub.0, R.sub.0 is a time integration value of a resistance between the electrodes from current passage start to the reference electrode force switching point T.sub.0 in the case where current passage is performed under a same condition as the main current passage when the parts to be welded have no disturbance, and T.sub.A is a time at which a time integration value of a resistance between the electrodes in the main current passage reaches R.sub.0.

2. The resistance spot welding method according to claim 1, wherein the reference electrode force switching point T.sub.0satisfies the following formula:
0.1×T.sub.m≤T.sub.0≤0.8×T.sub.m.

3. The resistance spot welding method according to claim 1, comprising: performing test welding; and performing actual welding including the main current passage, after the test welding, wherein in main current passage in the test welding, a time variation curve of an instantaneous amount of heat generated per unit volume and a cumulative amount of heat generated per unit volume that are calculated from an electrical property between the electrodes in forming an appropriate nugget by performing current passage by constant current control are stored, and in the main current passage in the actual welding, the time variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume that are stored in the main current passage in the test welding are set as a target, and a current passage amount is controlled according to the target.

4. A weld member production method comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to claim 1.

5. The resistance spot welding method according to claim 2, comprising: performing test welding; and performing actual welding including the main current passage, after the test welding, wherein in main current passage in the test welding, a time variation curve of an instantaneous amount of heat generated per unit volume and a cumulative amount of heat generated per unit volume that are calculated from an electrical property between the electrodes in forming an appropriate nugget by performing current passage by constant current control are stored, and in the main current passage in the actual welding, the time variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume that are stored in the main current passage in the test welding are set as a target, and a current passage amount is controlled according to the target.

6. A weld member production method comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to claim 2.

7. A weld member production method comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to claim 3.

8. A weld member production method comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to claim 5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In the accompanying drawings:

[0042] FIG. 1 is a diagram illustrating the relationship between the electrode force and time in main current passage and the relationship between the time integration value of the resistance between the electrodes and time in the main current passage when Formula (1) is satisfied (in the case where T.sub.A≤0.8×T.sub.0) in a resistance spot welding method according to one of the disclosed embodiments;

[0043] FIG. 2 is a diagram illustrating the relationship between the electrode force and time in the main current passage and the relationship between the time integration value of the resistance between the electrodes and time in the main current passage when Formula (2) is satisfied (in the case where 0.8×T.sub.0<T.sub.A≤T.sub.0 or 0.9×R.sub.0≤R.sub.A≤R.sub.0) in the resistance spot welding method according to one of the disclosed embodiments;

[0044] FIG. 3 is a diagram illustrating the relationship between the electrode force and time in the main current passage and the relationship between the time integration value of the resistance between the electrodes and time in the main current passage when Formula (3) is satisfied (in the case where R.sub.A<0.9×R.sub.0) in the resistance spot welding method according to one of the disclosed embodiments;

[0045] FIG. 4A is a diagram schematically illustrating an example of performing welding in a state of no disturbance;

[0046] FIG. 4B is a diagram schematically illustrating an example of performing welding in a state of no disturbance;

[0047] FIG. 5A is a diagram schematically illustrating an example of performing welding on a sheet combination having a sheet gap;

[0048] FIG. 5B is a diagram schematically illustrating an example of performing welding on a sheet combination having a sheet gap;

[0049] FIG. 6A is a diagram schematically illustrating an example of performing welding on a sheet combination having existing welds; and

[0050] FIG. 6B is a diagram schematically illustrating an example of performing welding on a sheet combination having existing welds.

DETAILED DESCRIPTION

[0051] One of the disclosed embodiments will be described below.

[0052] One of the disclosed embodiments is a resistance spot welding method of squeezing, by a pair of electrodes, parts to be welded which are a plurality of overlapping metal sheets and passing a current while applying an electrode force to join the parts to be welded, wherein main current passage includes two or more electrode force application steps including a first electrode force application step and a second electrode force application step following the first electrode force application step, an electrode force F.sub.1 in the first electrode force application step and an electrode force F.sub.2 in the second electrode force application step in the main current passage satisfy a relationship F.sub.1<F.sub.2, and an electrode force switching point T.sub.f from the first electrode force application step to the second electrode force application step in the main current passage (hereafter also referred to as “electrode force switching point T.sub.f”) is set to satisfy a predetermined relationship.

[0053] The electrode force switching point T.sub.f (and the below-described reference electrode force switching point T.sub.0) is the time at which the electrode force switching operation starts.

[0054] The electrode force switching point T.sub.f (and the below-described reference electrode force switching point T.sub.0) is expressed relative to the current passage start point of the main current passage (i.e. expressed as the time elapsed from the current passage start point of the main current passage). The same applies to the below-described T.sub.A (the time at which the time integration value of the resistance between the electrodes in the main current passage reaches R.sub.0), etc.

[0055] The resistance spot welding method according to one of the disclosed embodiments is particularly suitable for a sheet combination whose sheet thickness ratio ((the total thickness of the sheet combination)/(the sheet thickness of the thinnest metal sheet in the sheet combination)) is more than 3 and further suitable for a sheet combination whose sheet thickness ratio is 5 or more, for which it has been difficult to obtain a nugget of a required size between thin and thick sheets without expulsion regardless of a disturbance. The resistance spot welding method is effective for a sheet combination of two overlapping sheets as well.

[0056] The term “thin sheet” means a metal sheet with relatively small sheet thickness and the term “thick sheet” means a metal sheet with relatively large sheet thickness, of the steel sheets used in the sheet combination. The sheet thickness of a thin sheet is not greater than ¾ of that of a metal sheet (thick sheet) with the largest sheet thickness.

[0057] Any welding device that includes a pair of upper and lower electrodes and is capable of freely controlling each of the electrode force and the welding current during welding may be used in the resistance spot welding method according to one of the disclosed embodiments. The type (stationary, robot gun, etc.), the electrode shape, and the like are not limited.

[0058] The resistance spot welding method according to one of the disclosed embodiments will be described below.

[0059] (A) Main current passage (also referred to as “main current passage in actual welding” in order to be distinguished from main current passage in test welding (described later). The term “main current passage” when used alone denotes the main current passage in the actual welding and not the main current passage in the test welding. Herein, the “main current passage” denotes current passage for forming a nugget. The “actual welding” denotes a process of actually welding parts to be welded, which is to be distinguished from the test welding.)

[0060] For a sheet combination of three or more overlapping sheets with a high sheet thickness ratio, by dividing the main current passage for nugget formation into two or more electrode force application steps and satisfying the following relationship, i.e. setting the electrode force F.sub.1 in the first electrode force application step (hereafter also simply referred to as “F.sub.1”) to be less than the electrode force F.sub.2 in the second electrode force application step (hereafter also simply referred to as “F.sub.2”), heat generation between the thin and thick sheets is preferentially promoted in the first electrode force application step:

[0061] F.sub.1<F.sub.2.

[0062] Preferably, 1.1×F.sub.i≤F.sub.2. More preferably, 1.2×F.sub.1≤F.sub.2. Further preferably, 1.5×F.sub.1≤F.sub.2.

[0063] F.sub.1 and F.sub.2 may be set as appropriate depending on the materials, thicknesses, etc. of the metal sheets as the parts to be welded, as long as the foregoing relationship is satisfied.

[0064] For example, in the case of using a sheet combination of three or more overlapping sheets with a high sheet thickness ratio (e.g. a sheet combination of three overlapping sheets composed of two thick sheets (mild steel or 490 MPa to 2000 MPa-grade zinc or zinc alloy coated steel sheets or non-coated steel sheets of 0.8 mm to 3.0 mm in thickness) and one thin sheet (zinc or zinc alloy coated steel sheet or non-coated steel sheet (mild steel) of 0.5 mm to 2.0 mm in thickness), it is preferable that F.sub.1 is 1.0 kN to 6.0 kN and F.sub.2 is 2.0 kN to 10.0 kN.

[0065] In the case of using a typical sheet combination of two overlapping sheets, it is preferable that F.sub.1 is 1.0 kN to 5.0 kN and F.sub.2 is 2.0 kN to 7.0 kN.

[0066] In the resistance spot welding method according to one of the disclosed embodiments, it is important to set the timing of switching from F.sub.1 to F.sub.2, i.e. the electrode force switching point T.sub.f, so as to satisfy the following Formulas (1) to (3), depending on the time integration value of the resistance between the electrodes from the current passage start of the main current passage to when a predetermined time elapses:

[0067] in the case where T.sub.A≤0.8×T.sub.0,

T.sub.A≤T.sub.f<T.sub.0   (1)

[0068] in the case where 0.8×T.sub.0<T.sub.A≤T.sub.0 or 0.9×R.sub.0≤R.sub.A≤R.sub.0,

0.9×T.sub.0<T.sub.f<1.1×T.sub.0   (2)

[0069] in the case where R.sub.A<0.9×R.sub.0,

T.sub.0<T.sub.fT.sub.0+2×(R.sub.0−R.sub.A)/R.sub.0×T.sub.m   (.sup.3)

[0070] where T.sub.0 is the reference electrode force switching point from the first electrode force application step to the second electrode force application step, T.sub.m is the total welding time in the main current passage, R.sub.A is the time integration value of the resistance between the electrodes from the current passage start of the main current passage to the reference electrode force switching point T.sub.0, R.sub.0 is the time integration value of the resistance between the electrodes from the current passage start to the reference electrode force switching point T.sub.0in the case where current passage is performed under the same condition as the main current passage when the parts to be welded have no disturbance, and T.sub.A is the time at which the time integration value of the resistance between the electrodes in the main current passage reaches R.sub.0.

[0071] In the case where T.sub.A≤0.8×T.sub.0, that is, in the case where R.sub.A is expected to be greater than R.sub.0 by a certain amount (see FIG. 1), in the first electrode force application step, the contact diameter between the metal sheets is smaller than in a state of no disturbance due to a sheet gap. In other words, desired heat generation between the thin and thick sheets is likely to be achieved in a shorter time. In such a case, it is effective to advance the timing of switching from F.sub.1 to F.sub.2. Specifically, it is effective to set the electrode force switching point T.sub.f so as to satisfy the foregoing Formula (1).

[0072] In the case where R.sub.A<0.9×R.sub.0, that is, in the case where R.sub.A is less than R.sub.0 by a certain amount (see FIG. 3), heat generation between the thin and thick sheets is likely to be insufficient due to current shunting or the like. In such a case, it is effective to delay the timing of switching from F.sub.1 to F.sub.2. Specifically, it is effective to set the electrode force switching point T.sub.f so as to satisfy the foregoing Formula (3), to further promote heat generation between the thin and thick sheets.

[0073] In the case where 0.8×T.sub.0<T.sub.A≤T.sub.0 or 0.9×R.sub.0≤R.sub.A≤R.sub.0 (see FIG. 2), that is, in the case where R.sub.A is (or is expected to be) approximately equal to R.sub.0, the effect of a disturbance is unlikely to be significant. In such a case, the electrode force switching point T.sub.f is set so as to satisfy the foregoing Formula (2).

[0074] Thus, in the resistance spot welding method according to one of the disclosed embodiments, it is important to set the electrode force switching point T.sub.f so as to satisfy Formulas (1) to (3) depending on, for example, the time integration value of the resistance between the electrodes from the current passage start of the main current passage to when the predetermined time elapses.

[0075] As Formulas (1) to (3), it is preferable to satisfy the following Formulas (1)′ to (3)′ respectively:

[0076] in the case where T.sub.A≤0.8≤T.sub.0,

T.sub.A≤T.sub.f≤0.95×T.sub.0   (1)′

[0077] in the case where 0.8×T.sub.0<T.sub.A≤T.sub.0 or 0.9×R.sub.0≤R.sub.A≤R.sub.0,

0.95×T.sub.0<T.sub.f<1.05×T.sub.0   (2)′

[0078] in the case where R.sub.A<0.9×R.sub.0,

1.05×T.sub.0≤T.sub.f≤T.sub.0+2×(R.sub.0−R.sub.A)/R.sub.0×T.sub.m   (3)′.

[0079] For example, the time integration value R.sub.0 of the resistance between the electrodes from the current passage start to the reference electrode force switching point T.sub.0 in the case where current passage is performed under the same condition as the main current passage when the parts to be welded have no disturbance may be obtained by separately preparing parts to be welded composed of metal sheets of the same sheet thicknesses and materials as in the main current passage and having no disturbance and conducting a preliminary welding test of welding the parts to be welded under the same condition as the main current passage.

[0080] In the case of performing the below-described test welding, the time integration value of the resistance between the electrodes from the current passage start to the reference electrode force switching point T.sub.0in the main current passage in the test welding may be R.sub.0.

[0081] The reference electrode force switching point T.sub.0 (ms) from the first electrode force application step to the second electrode force application step may be set as appropriate depending on, for example, the materials and thicknesses of the metal sheets as the parts to be welded, but is preferably set using the total welding time T.sub.m (ms) in the main current passage so as to satisfy the following formula:

0.1×T.sub.m≤T.sub.0≤0.8×T.sub.m.

[0082] If T.sub.0 is less than 0.1×T.sub.m, there is a possibility that the effect of a disturbance cannot be mitigated effectively by controlling the electrode force switching timing. If T.sub.0 is more than 0.8×T.sub.m, too, there is a possibility that the effect of a disturbance cannot be mitigated effectively by controlling the electrode force switching timing. T.sub.0 is therefore preferably 0.1×T.sub.m or more and 0.8×T.sub.m or less.

[0083] T.sub.0 is more preferably 0.15×T.sub.m, or more, and further preferably 0.2×T.sub.m or more. T.sub.0 is more preferably 0.7×T.sub.m or less, and further preferably 0.5×T.sub.m or less.

[0084] The total welding time T.sub.m (ms) in the main current passage may be set as appropriate depending on, for example, the materials and thicknesses of the metal sheets as the parts to be welded.

[0085] For example, in the case of using a sheet combination of three or more overlapping sheets with a high sheet thickness ratio as mentioned above, T.sub.m is preferably 120 ms to 1000 ms. In the case of using a typical sheet combination of two overlapping sheets, T.sub.m is preferably 80 ms to 800 ms.

[0086] In the case where the main current passage is divided into two or more current passage steps and a cooling time is provided between the current passage steps, the total welding time in the main current passage includes the cooling time between the current passage steps.

[0087] The main current passage may be performed by constant current control. Alternatively, after performing the below-described test welding, adaptive control welding of controlling the current passage amount according to the target set in the test welding may be performed.

[0088] In the case of constant current control, the welding current may be set as appropriate depending on, for example, the materials and thicknesses of the metal sheets as the parts to be welded. The main current passage may be divided into two or more current passage steps, and a cooling time may be provided between the current passage steps.

[0089] The timing of dividing the current passage may be the same as or different from the timing of dividing the electrode force application. The point of switching the current value from the first current passage step to the second current passage step (i.e. the timing of dividing the current passage) in the main current passage need not be changed according to the change of the electrode force switching point in the main current passage. The same applies to the below-described adaptive control welding.

[0090] For example, in the case of welding a typical sheet combination of two overlapping sheets by one current passage step, the current value is preferably 4.0 kA to 12.0 kA.

[0091] In the case of performing welding by two or more current passage steps obtained by dividing the current passage, it is preferable that the current value and the welding time in the first current passage step are 4.0 kA to 14.0 kA and 20 ms to 400 ms respectively and the current value and the welding time in the second current passage step are 3.0 kA to 12.0 kA and 40 ms to 800 ms respectively. Particularly in the case of welding a sheet combination of three or more overlapping sheets with a high sheet thickness ratio as mentioned above, it is preferable that the current value in the first current passage step is greater than the current value in the second current passage step. In the case where a cooling time is provided between the first current passage step and the second current passage step, the cooling time is preferably 10 ms to 400 ms.

[0092] In the case of adaptive control welding, welding is performed according to the target (the time variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated) obtained as a result of the below-described test welding. If the amount of time variation of the instantaneous amount of heat generated per unit volume follows the time variation curve, the welding is continued without change and completed. If the amount of time variation of the instantaneous amount of heat generated per unit volume differs from the time variation curve, the current passage amount is controlled in order to compensate for the difference within a remaining welding time so that the cumulative amount of heat generated per unit volume in the actual welding matches the cumulative amount of heat generated per unit volume set as the target.

[0093] In the case of adaptive control welding, too, the main current passage may be divided into two or more current passage steps, and adaptive control welding may be performed for each current passage step.

[0094] In detail, the main current passage in the actual welding and the main current passage in the test welding are each divided into two or more current passage steps so as to correspond to each other.

[0095] Welding is then performed according to the target (the time variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated) for each current passage step obtained as a result of the test welding. If the amount of time variation of the instantaneous amount of heat generated per unit volume differs from the time variation curve in any current passage step, the current passage amount is controlled in order to compensate for the difference within a remaining welding time in the current passage step so that the cumulative amount of heat generated per unit volume in the current passage step matches the cumulative amount of heat generated per unit volume in the current passage step in the test welding.

[0096] The method of calculating the amount of heat generated is not limited. JP H11-33743 A discloses an example of the method, which may be used herein. The following is the procedure of calculating the amount q of heat generated per unit volume and per unit time and the cumulative amount Q of heat generated per unit volume according to this method. Let t be the total thickness of the parts to be welded, r be the electrical resistivity of the parts to be welded, V be the voltage between the electrodes, I be the welding current, and S be the contact area of the electrodes and the parts to be welded. In this case, the welding current passes through a columnar portion whose cross-sectional area is S and thickness is t, to generate heat by resistance. The amount q of heat generated per unit volume and per unit time in the columnar portion is given by the following Formula (4):

q=(V.Math.I)/(S.Math.t)   (4).

[0097] The electrical resistance R of the columnar portion is given by the following Formula (5):

R=(r.Math.t)/S   (.sup.5).

[0098] Solving Formula (5) for S and substituting the solution into Formula (4) yields the amount q of heat generated as indicated by the following Formula (6):

q=(V.Math.I.Math.R)/(r.Math.t.sup.2)=(V.sup.2)/(r.Math.t.sup.2)   (6).

[0099] As is clear from Formula (6), the amount q of heat generated per unit volume and per unit time can be calculated from the voltage V between the electrodes, the total thickness t of the parts to be welded, and the electrical resistivity r of the parts to be welded, and is not affected by the contact area S of the electrodes and the parts to be welded. Although the amount of heat generated is calculated from the voltage V between the electrodes in Formula (6), the amount q of heat generated may be calculated from the interelectrode current I. The contact area S of the electrodes and the parts to be welded need not be used in this case, either. By cumulating the amount q of heat generated per unit volume and per unit time for the welding time, the cumulative amount Q of heat generated per unit volume for the welding is obtained. As is clear from Formula (6), the cumulative amount Q of heat generated per unit volume can also be calculated without using the contact area S of the electrodes and the parts to be welded.

[0100] Although the above describes the case of calculating the cumulative amount Q of heat generated by the method described in JP H11-33743 A, the cumulative amount Q may be calculated by any other method.

[0101] (B) Test welding

[0102] In the case of performing the main current passage in the actual welding by adaptive control welding, the test welding is performed before the actual welding. In the main current passage in the test welding, the time variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume that are calculated from the electrical property between the electrodes in forming an appropriate nugget by performing current passage by constant current control are stored.

[0103] In detail, in the test welding, a preliminary welding test with the same steel types and thicknesses as the parts to be welded in the actual welding is performed with various conditions by constant current control in a state without a sheet gap or current shunting to an existing weld, to find an optimal condition in the test welding.

[0104] Current passage is then performed under this condition, and the time variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume that are calculated from the electrical property between the electrodes during the current passage are stored as the target in the actual welding. Herein, the “electrical property between the electrodes” means the resistance between the electrodes or the voltage between the electrodes.

[0105] The main current passage in the test welding may be divided into two or more current passage steps, and adaptive control welding may be performed for each current passage step in the actual welding, as mentioned above.

[0106] In the case of welding a sheet combination of three or more overlapping sheets with a high sheet thickness ratio as mentioned above, it is preferable that the current value in the first current passage step is greater than the current value in the second current passage step in the test welding, too.

[0107] (C) Other modifications

[0108] Preliminary current passage for stabilizing the contact diameter may be performed before the main current passage (the main current passage in the actual welding and/or the test welding) for nugget formation, and subsequent current passage for subsequent heat treatment may be performed. The preliminary current passage and the subsequent current passage may be performed by constant current control, or performed in an upslope or downslope current pattern.

[0109] A cooling time may be provided between the preliminary current passage and the main current passage and between the main current passage and the subsequent current passage.

[0110] The parts to be welded are not limited. The resistance spot welding method may be used for welding of steel sheets and coated steel sheets having various strengths from mild steel to ultra high tensile strength steel and light metal sheets of aluminum alloys and the like. The resistance spot welding method may also be used for a sheet combination of three or more overlapping steel sheets.

[0111] By joining a plurality of overlapping metal sheets by the resistance spot welding method described above, various weld members, in particular weld members of automotive parts and the like, can be produced while stably ensuring a desired nugget diameter by effectively responding to variations in the disturbance state.

EXAMPLES

[0112] The presently disclosed techniques will be described below by way of examples. The conditions in the examples are one example of conditions employed to determine the operability and effects of the presently disclosed techniques, and the present disclosure is not limited to such example of conditions. Various conditions can be used in the present disclosure as long as the object of the present disclosure is fulfilled, without departing from the scope of the present disclosure.

[0113] Actual welding (main current passage) was performed for each sheet combination of metal sheets listed in Table 1 under the conditions listed in Tables 3 and 4 in the states illustrated in FIGS. 4A to 6B, to produce weld joints.

[0114] In FIGS. 5A and 5B, spacers 15 were inserted between metal sheets, and a sheet combination was clamped from above and below (not illustrated) to create a sheet gap of any of various thicknesses. The distance between the spacers was 60 mm in each case.

[0115] In FIGS. 6A and 6B, there were two existing welds 16, and the welding position (the center between the electrodes) was adjusted to be a midpoint between the existing welds (i.e. the distance L from each existing weld was equal). The nugget diameter of each existing weld was 4√t′ (mm) (where t′ is the sheet thickness (mm) of the thinnest metal sheet in the sheet combination).

[0116] In some examples, before actual welding, test welding was performed under the conditions listed in Table 2 in a state of no disturbance illustrated in FIGS. 4A and 4B, and the time variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume in the main current passage in the test welding were stored.

[0117] Moreover, the time integration value of the resistance between the electrodes from the current passage start to the reference electrode force switching point T.sub.0in the main current passage in the test welding was measured and taken to be R.sub.0.

[0118] In each example in which current passage was performed by constant current control, parts to be welded composed of metal sheets of the same sheet thicknesses and materials as in the main current passage and having no disturbance were separately prepared, and a preliminary welding test of welding the parts to be welded under the same condition as the actual welding was performed to obtain R.sub.0.

[0119] R.sub.0 thus obtained is listed in Table 3.

[0120] For each produced weld joint, the weld was cut and etched in section, and then observed with an optical microscope to evaluate the weld joint in the following three levels A, B, and F based on the nugget diameter and whether expulsion occurred. For a sheet combination of three overlapping sheets, the evaluation was made using the diameter of a nugget formed between the thinnest metal sheet 11 on the outer side and the metal sheet 12 adjacent to the metal sheet 11. The evaluation results are listed in Table 4.

[0121] A: The nugget diameter was 4.5√t′ (mm) or more (where t′ is the sheet thickness (mm) of the thinnest metal sheet in the sheet combination) and no expulsion occurred regardless of a disturbance.

[0122] B: The nugget diameter was 4√t′ (mm) or more and no expulsion occurred regardless of a disturbance (excluding the case A).

[0123] F: The nugget diameter was less than 4√t′ (mm) and/or expulsion occurred depending on a disturbance.

TABLE-US-00001 TABLE 1 Sheet Metal sheet of Metal sheet of Metal sheet of combination reference sign 11 reference sign 12 reference sign 13 4√t′ 4.5√t′ ID in the drawings in the drawings in the drawings (mm) (mm) A 270 MPa-grade GA 1470 MPa-grade GA 1470 MPa-grade GA 3.35 3.76 steel sheet (sheet steel sheet (sheet steel sheet (sheet thickness: 0.7 mm) thickness: 1.6 mm) thickness: 1.6 mm) B 270 MPa-grade GA 1470 MPa-grade 1470 MPa-grade GA 3.58 4.02 steel sheet (sheet cold-rolled steel sheet (sheet thickness: 0.8 mm) steel sheet (sheet thickness: 1.4 mm) thickness: 1.6 mm) C 1470 MPa-grade GA 1470 MPa-grade GA 4.73 5.32 steel sheet (sheet steel sheet (sheet thickness: 1.4 mm) thickness: 1.4 mm)

TABLE-US-00002 TABLE 2 Test welding condition Electrode force Current passage condition application condition First current Second current Switching passage step passage step Sheet point Current Welding Cooling Current Welding combination F.sub.1′*.sup.1 F2*.sup.2 from to F.sub.2′ value tine time value time No. ID (kN) (kN) (ms) (kA) (ms) (ms) (kA) (ms) Remarks 1 1-1 A 3.0 5.0 80 8.0 80 40 6.5 300 Example 1-2 1-3 1-4 2 2-1 B — Example 2-2 2-3 2-4 3 3-1 C 3.0 5.0 100 6.5 100 — 6.5 180 Example 3-2 3-3 3-4 4 4-1 A — Comparative 4-2 Example 4-3 4-4 *.sup.1Electrode force in first electrode force application step in main current passage in test welding *.sup.2Electrode force in second electrode force application step in main current passage in test welding

TABLE-US-00003 TABLE 3 Actual welding condition Electrode force switching Electrode force Sheet timing application condition Appropriate range of combination change F.sub.1 F.sub.2 T.sub.f T.sub.0 T.sub.m T.sub.A R.sub.0 R.sub.A T.sub.f according to No. ID Disturbance state control (kN) (kN) (ms) (ms) (ms) T.sub.0/T.sub.m (ms) (Ω .Math. ms) (Ω .Math. ms) R.sub.A/R.sub.0 Formulas (1) to (3) Remarks 1 1-1 A None Applied 3.0 5.0 80 80 420 0.19 78 0.014 0.015 1.07 More than 72 to less Formula (2) Example than 88 1-2 Sheet gap (tg = 0.5 mm) 60 50 0.018 1.29 50 to less than 80 Formula (1) 1-3 Sheet gap (tg = 1 mm) 40 34 0.022 1.57 34 to less than 80 Formula (1) 1-4 Existing weld (L = 10 mm) 160 150 0.011 0.79 More than 80 to 260 Formula (3) 2 2-1 B None Applied 3.5 5.0 60 60 440 0.14 58 0.010 0.010 1.00 More than 54 to less Formula (2) Example than 66 2-2 Sheet gap (tg = 0.5 mm) 50 46 0.014 1.40 46 to less than 60 Formula (1) 2-3 Sheet gap (tg = 1 mm) 40 38 0.017 1.70 38 to less than 60 Formula (1) 24 Existing weld (L = 10 mm) 110 90 0.008 0.80 More than 60 to 236 Formula (3) 3 3-1 C None Applied 3.0 5.0 97 100 280 0.36 92 0.027 0.029 1.07 More than 90 to less Formula (2) Example than 110 3-2 Sheet gap (tg = 0.5 mm) 80 74 0.035 1.30 74 to less than 100 Formula (1) 3-3 Sheet gap (tg = 1 mm) 70 62 0.035 1.30 62 to less than 100 Formula (1) 34 Existing weld (L = 10 mm) 140 130 0.020 0.74 More than 100 to 245 Formula (3) 4 4-1 A None Not applied 3.0 5.0 80 80 420 0.19 78 0.014 0.015 1.07 More than 72 to less Formula (2) Comparative than 88 Example 4-2 Sheet gap (tg = 0.5 mm) 80 56 0.017 1.21 56 to less than 80 Formula (1) 4-3 Sheet gap (tg = 1 mm) 80 42 0.020 1.43 42 to less than 80 Formula (1) 44 Existing weld (L = 10 mm) 80 140 0.012 0.86 More than 80 to 200 Formula (3)

TABLE-US-00004 TABLE 4 Actual welding condition Current passage condition First current Second current passage step passage step Evaluation result Sheet Current Welding Cooling Current Welding Nugget combination Current passage value time time value time diameter No. ID method (kA) (ms) (ms) (kA) (ms) (mm) Expulsion Evaluation Remarks 1 1-1 A Adaptive control — 80 40 — 300 3.9 Not occurred A Example 1-2 4.0 Not occurred 1-3 3.9 Not occurred 1-4 4.2 Not occurred 2 2-1 B Constant 7.5 60 60 6.0 320 4.3 Not occurred B Example 2-2 current control 4.1 Not occurred 2-3 3.7 Not occurred 2-4 4.1 Not occurred 3 3-1 C Adaptive control — 100 — — 180 5.5 Not occurred A Example 3-2 5.4 Not occurred 3-3 5.6 Not occurred 3-4 5.5 Not occurred 4 4-1 A Constant 8.0 80 40 6.5 300 3.9 Not occurred F Comparative 4-2 current control 3.1 Occurred Example 4-3 2.9 Occurred 4-4 3.2 Not occurred

[0124] In each Example, a sufficient nugget diameter was obtained without expulsion regardless of a disturbance.

[0125] In each Comparative Example, a sufficient nugget diameter was not obtained and/or expulsion occurred depending on a disturbance.

REFERENCE SIGNS LIST

[0126] 11, 12, 13 metal sheet

[0127] 14 electrode

[0128] 15 spacer

[0129] 16 existing weld